CNC Grinding System | CNC Grinding Control Systems

Control systems for CNC grinding

John Liverton

Perhaps the most exciting developments in CNC grinding are in the controls themselves. Here, one builder explains just how far this technology has come.

The development of CNC systems for precision grinding machines has revolutionized their use both in the toolroom and in the production environment. Today's very versatile software enables fully-automatic, precision grinding of simple or complex parts as one-offs or in batch production. Profiles can be generated directly on the workpiece or performed on the grinding wheel. Various dressing and grinding cycles can be combined into a single automatic sequence that grinds a workpiece complete with minimum operator intervention.

In addition to the grinding and wheel-dressing operations, the same CNC system is used to control a wide range of ancillary equipment, such as rotary tables, gaging units, automatic loading devices, and so on. The result is a production unit capable of high levels of productivity, accuracy and repeatability.

A Case in Point

A factor that delayed the general acceptance of CNC grinding, particularly in smaller organizations, was the need to learn and understand complex programming languages. The requirement for a programming system that is easy to understand on the shop floor was obvious, and that capability is now readily available in a range of grinding machines. Jones & Shipman worked closely with one of the leading CNC suppliers, Allen Bradley, to achieve this with its Format range of software running on the Allen Bradley 8600 series. Certain Format machines now also use GE Fanuc's Series 18 system.

In either case, operational software is menu-driven and incorporates clear graphic displays for both grinding and wheel-dressing cycles. When necessary, an operator can use his skill and experience to override the control program, for example, to give an extra wheel-dressing cycle or gage the workpiece.

Another advantage to the user is when similar control systems can be applied to different processes. This is demonstrated by the close similarity of the surface and cylindrical grinding programming procedures illustrated in Figure 1. Thus, it is relatively easy for an operator trained on one machine to operate the other, allowing better utilization of the production machine and greater flexibility in the scheduling of work and labor. And where required, special programs can be written for non-standard applications to supplement the standard features.

Process Specific

Today's CNC grinding machines are designed as a single integrated unit, rather than adding a control system to an existing machine, which was the normal practice when control systems were first applied to the grinding process. Unique software is developed by engineers experienced in the grinding process. Programming is logical and easily understood and carried out by machine operators on the shop floor, or off-line by production engineers. Rapid setups are possible, some as quick as five minutes.

The control system simply asks the operator for the data required for grinding the workpiece on a particular machine. The software then calculates and displays the optimum grinding parameters, including incremental or plunge feed rates, divided into coarse, medium and fine sections; table traverse speeds; spark-outs; and (in the case of cylindrical grinding) speeds at which the workpiece should be rotated. The operator then has the opportunity to either accept or edit these parameters.

An example of the use of combined cycles to give a highly versatile operation is shown in Figure 2. This is on a Format 15P ID/OD cylindrical grinding machine, which has a powered and programmable wheelhead swivel and is capable of both external and internal grinding. Seven combined cycles are used in an automatic sequence that includes grinding four external diameters, three external faces, two internal diameters, and one internal radius, all in one workpiece chucking.

The machine is fitted with a 350-mm diameter external wheel and a 38-mm diameter internal wheel. A quick setting in-process gage is used on one of the diameters.

Contouring Control

One of the more recent developments, made available through the introduction of CNC, is contour generation grinding on both surface and cylindrical grinders, that enables the grinding of profiles many times wider than the wheel width, using simultaneous movements of the machine's two axes. Such profiles can include a combination of radii, angles, sharp corners, parabolic or inverse curves, as shown in Figure 3. In some applications, contour generation on a surface grinding machine gives better results than wire erosion methods for both cycle time and surface finish.

New Possibilities

With advancing microprocessing technology, and with 32-bit processors in particular, have come much more powerful CNCs. This, in turn, allows more sophisticated control systems, both in terms of hardware and software.

For example, Jones & Shipman now has a cylindrical grinding control system that takes advantage of the enhanced capabilities of the new Allen Bradley Series 9/360 control. Compared with previous models, the 32-bit 9/360 is considerably more powerful with increased storage capacity on hard disk drive, and incorporates specially-developed, easy-to-use color graphic screens and menus to further simplify setup and programming. The system can control up to 12 axes, which means many items of ancillary equipment can be integrated when required, such as automatic loading/unloading, post-process gaging with SPC, and so on.

While the machine is carrying out an automatic grinding sequence on one component, the operator is able to program a new component, edit an existing program, or exchange information with external computer systems running applications such as CAD or work scheduling. The 9/360 control incorporates a personal computer with an MS-DOS operating system that enables it to be used as a workstation in a network. The MS-DOS partition can be accessed while grinding so, for example, it could be used for such real-time functions as adaptive control. Communication between the control and the rest of the machine is by high-speed fiber optics.

Programs are developed on the powerful graphic editor by selecting macros and geometric elements that appear on the screen, and adding the required part dimensions. A comprehensive "Help" facility is available to help the operator with all the necessary information about the current process, as shown in Figure 4.

In addition to the standard surface grinding machines, advanced new CNC systems with 32-bit processors bring new advantages to the high-tech range of creep-feed grinding machines as well. User-friendly software--incorporating menus, macros and graphical programming--further simplifies and speeds up part programming.

Linear scales are fitted to all four axes to give highly accurate positional accuracy. This is particularly important on applications where interpolation is used to generate profiled components. This permits resolution of 0.0001 mm on all axes.

Development of CNC systems for grinding machines is an on-going process aimed at providing additional facilities and simplifying programming still further. At the same time, quality and productivity are improved, and optimum use made of existing skilled labor.